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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

Four Cisco routers (R1, R2, R3, and R4) are connected to a single Layer 2 Ethernet switch on the 10.1.1.0/24 broadcast multi-access network segment. All four routers run OSPFv2 in Area 0 and are powered on simultaneously. Their interface OSPF priorities and OSPF Router IDs are configured as follows:

- R1: Router ID 192.168.100.1, Interface Priority = 0
- R2: Router ID 10.1.1.20, Interface Priority = 100
- R3: Router ID 172.16.50.1, Interface Priority = 100
- R4: Router ID 10.1.1.40, Interface Priority = 10

Which router is elected as the Designated Router (DR) and which router is elected as the Backup Designated Router (BDR) for this multi-access segment?

  1. R3 is elected as DR, and R2 is elected as BDR.Answer
  2. B
    R1 is elected as DR, and R3 is elected as BDR.
  3. C
    R2 is elected as DR, and R3 is elected as BDR.
  4. D
    R3 is elected as DR, and R1 is elected as BDR.

Answer

R3 is elected as Designated Router (DR), and R2 is elected as Backup Designated Router (BDR).
The correct response identifies R3 as DR and R2 as BDR. In OSPFv2 broadcast multi-access networks, any router with an interface priority of 0 is strictly excluded from becoming a DR or BDR (relegated to DROTHER status). Thus, R1 is ineligible despite having the highest Router ID. Among the eligible routers (R2, R3, and R4), R2 and R3 share the highest priority (100). The tie is broken by comparing Router IDs; R3 (172.16.50.1) is higher than R2 (10.1.1.20), making R3 the DR. R2 is then elected as BDR because its priority (100) is higher than R4's priority (10).

Step-by-Step Solution

1
Filter out disqualified routers based on OSPF interface priority.
R1 has an OSPF priority of 0, which makes it ineligible to participate in DR/BDR elections (it remains DROTHER). Eligible candidates are R2 (Priority 100), R3 (Priority 100), and R4 (Priority 10).
An interface priority of 0 explicitly prevents a router from becoming a DR or BDR regardless of its Router ID.
2
Determine the Designated Router (DR) using priority and Router ID rules.
R2 and R3 have the highest priority value (100). Comparing Router IDs, R3's Router ID (172.16.50.1) is numerically higher than R2's Router ID (10.1.1.20). R3 is selected as DR.
OSPF DR election first chooses the candidate with the highest priority. If priorities tie, it chooses the candidate with the highest Router ID.
3
Determine the Backup Designated Router (BDR) from the remaining eligible candidates.
Remaining candidates are R2 (Priority 100) and R4 (Priority 10). R2 has the higher priority, so R2 is selected as BDR.
After DR selection, the BDR is chosen using the same highest-priority and highest-Router-ID criteria among remaining eligible routers.

Key Concept

OSPFv2 DR/BDR Election Precedence and Priority 0 Disqualification
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Topics

OSPFv2 Network Types and DR/BDR Selection

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